The invention relates to an actuator assembly for a vehicle brake as well as a vehicle brake with such an actuator assembly.
In the field of motor vehicles there are tendencies to abandon hydraulically operated cylinder/piston arrangements and to realize electrically operated vehicle brakes (brake-by-wire).
Generally, each vehicle wheel of each motor vehicle is provided with a braking unit which is formed by a disk or drum brake as well as a brake piston. The brake piston acts hydraulically or pneumatically on the disk or drum brake. Each brake piston is either associated with an individual booster, or all brake pistons are controlled by a common brake booster. The behaviour of one or several vehicle wheels with respect to deceleration or acceleration, respectively, is monitored via an electronic control unit. In addition, further parameters of the braking operation, e.g. the actuation travel of the brake pedal, the brake pedal actuation speed, the brake pedal actuation force, are sensed and evaluated. From this (and, if required, additional) data the electronic control unit determines a necessary brake force assistance and activates the brake booster of the respective brake or modulates the brake force of the common brake booster, which is available for all braking units, whereupon the hydraulic brake cylinder pressure which is solely applied by the pedal actuation force is amplified accordingly.
For safety reasons, all of these known braking systems are equipped with a direct hydraulic connection between the pedalactuated master cylinder and the brake pistons besides an electronic activation, in order to enable the brake to continue to be actuated even in the case of a failure of the electronic system.
From DE 35 45 800 A1 a control device for vehicle wheels is known wherein a wheel cylinder and a brake cylinder are arranged in the brake caliper of each wheel of a motor vehicle. A piezoelectric actuation device is arranged in the brake caliper in such a manner that it is connected with the wheel cylinder. Upon the application of an electric voltage to the piezoelectric actuation device, the piston within a sleeve is displaced and changes the volume of the hydraulic fluid in the wheel cylinder. From a master cylinder brake fluid is supplied via a fluid duct in the brake caliper to the wheel cylinder, with said fluid duct being adapted for opening and closing by means of a control valve. When a high pressure prevails in the brake cylinder the piezoelectric actuation device expands, and the control valve closes. Subsequently, the electric voltage at the piezoelectric actuation device is reduced, whereby the pressure in the wheel cylinder is reduced. The piston which is guided within the sleeve must be supported in a fluid-tight and low-friction manner in order to enable a low-delay actuation of the brake piston. This causes high manufacturing costs and implies expensive assembly.
A similar control device for vehicle brakes is known from DE 36 00 733 A1, wherein a brake piston is arranged within the wheel cylinder, in whose interior a piezoelectric piston device is provided. Here, the brake piston (together with the piezoelectric piston device) is caused to carry out a feed motion of the brake by hydraulic fluid via a master cylinder. Thereby the brake piston together with the piezoelectric piston device moves under the action of the hydraulic fluid in the wheel cylinder. In addition to the hydraulically induced feed motion, the piezoelectric piston device which acts immediately on a brake lining can be activated. With this arrangement, too, guidance and support of the piezoelectric piston device in the brake piston is crucial with respect to the fluid-tight sealing and the low-wear and low-inertia guidance and support of same.
In the case of electrically operated brakes, however, the fact that the required force-travel characteristic (increasing force with increasing feed motion) of the force acting on friction element is not provided by the known electrically operatable actuators constitutes a problem.
The piezoelectric actuation means known from DE 35 45 800 A1 features an arrangement whose functioning capability is only very limited because the maximum travel of the piezoelectric element (and thus the displaced volume of the hydraulic fluid) is far too small. Moreover, a piezoelectric element has only a minimum force output, i.e. the variation of its characteristic is opposite to that of the desired force-travel characteristic.
The invention is therefore based on the object to provide an electrically operatable actuator assembly for a vehicle brake which does not have these drawbacks. This object is solved by the actuator assembly as described herein. Further characteristics, advantages and properties of the invention will be explained by means of the following description with reference to the drawings.